1. Field of Invention
The present invention relates to technology for converting an AC voltage to a DC voltage, more particularly relates to technology built into a switching power supply device and generating a desired power supply voltage based on an electric signal excited in a secondary winding of a transformer.
2. Description of the Related Art
In a switching power supply device, sometimes a drive circuit for operating the switching element and an auxiliary power supply circuit for supplying electric power to the control circuit are provided. An example of this auxiliary power supply circuit will be described related to FIG. 1. FIG. 1 is a diagram showing a circuit configuration of a forward type switching power supply device in related art. In FIG. 1, in addition to a basic configuration of the forward type switching power supply device, an auxiliary power supply circuit generating a power supply voltage supplied to a control circuit node (not shown) for driving a rectification circuit on the secondary side of a transformer T1 is shown.
The auxiliary power supply circuit shown in FIG. 1 includes a diode D100, capacitors C200 and C300, a transistor Q100, and a resistor R100 and generates a targeted power supply voltage Vcc from an emitter of the transistor Q100. In the auxiliary power supply circuit of FIG. 1, a pulse width modulation signal using an input voltage V1 as a peak voltage is given to the transformer T1 under the control of a switch element M300. Usually, in order to reduce the stress of a load at the time of activation of the power source, the switching power supply device performs a soft start gradually raising an output voltage until a prescribed value is reached. In this soft start, the PWM signal given to the transformer T1 gradually prolongs a conductive time of the switch element M300, that is, gradually enlarges a duty ratio.
In the auxiliary power supply circuit of FIG. 1, when a voltage Vs of one end of the secondary winding of the transformer T1 is positive, a base current is supplied to the transistor Q100 through the diode D100, the transistor Q100 turns on, and the desired power supply voltage Vcc is generated on both ends of the capacitor C300.
In the auxiliary power supply circuit shown in FIG. 1, however, when the voltage Vs of one end of the secondary winding of the transformer T1 becomes positive, it quickly turns on the transistor Q100, therefore the response of the power supply voltage Vcc is fast, so overshoot of an output voltage VO due to a delay of the power supply voltage Vcc can be reduced. However, after the power supply voltage reaches the prescribed value, there is a disadvantage of a large loss of the electric power by the transistor Q100. Therefore, an auxiliary power supply circuit with a high efficiency cannot be configured in the switching power supply device.